Thermally cured coatings are increasingly used industrially, particularly in the rapidly growing pre-coated sheet metal industry. The steel coils are coated with one or more coating layers and sold as pre-coated sheet metal to building, OEM and transportation industry. One layer of the coating system is often a thermally cured organic coating applied with roller application techniques. The application is normally performed with a liquid coating to form a wet-film, which then is cured thermally in a convection oven. One of the major drawbacks with this technique is that large quantities of organic solvents are needed to reduce the viscosity of the coating so it can be applied on the coil. The solvent is then evaporated during the drying and fumes incinerated to avoid solvent emissions. The use of solvents is not only an environmental problem, but does also add a significant cost to the coating since it must be taken care of in one-way or another. A significant improvement would be obtained if instead the solvent could be incorporated into the final dry coating. The same reasoning as above can also be applied to other thermally cured organic coatings where solvents are used to adjust the viscosity.
The demands on a reactive diluent can be listed as follows:                Good solvency of the intended resin system.        Low viscosity.        Low volatility.        The diluent should have a reactive group that allows it to be incorporated into the dry film.        The reactive diluent should react under the intended curing conditions (temperature, atmosphere, cure speed, etc.)        The reactive diluent should not introduce inappropriate physical properties to coating for the intended application.        
One group of monomers/molecules that have a long history in organic coatings is vegetable oils and derivatives thereof (Derksen et al). These monomers are traditionally used in air drying coating where unsaturations in the fatty acids react via an oxidation reaction to form a thermoset network. Examples of these air-drying systems are linseed oil coatings and air-drying alkyds. Other fatty acid systems are based on other functional groups such as hydroxyl groups in castor oil.
The use of fatty acid esters as reactive diluents in order to reduce or completely replace the volatile organic solvents in coatings has been described previously in many patent applications.
In EP 685543 DE 3803141, DE 3701410 esters of unsaturated fatty acids and various alcohols are used as reactive diluents. In DE 4129528 and EP 357128 the reactive diluents consist of fatty acid esters of various unsaturated alkyl ether alcohols. EP 305007 and EP 305006 employ an ester consisting of both an unsaturated fatty acid and unsaturated alcohol moiety as a reactive diluent. GB 2190672 use a reactive diluent that combines unsaturated fatty acids with unsaturated allyl polyols in an ester. Although these reports claim the use of reactive diluents in a multitude of binder and resins systems, in all these cases the incorporation of the reactive diluent is obtained via a reaction of unsaturated bonds in the diluent with the binder and the diluent molecules. The reaction is typically cured by air-drying.
In DE 19533168 and U.S. Pat. No. 4,877,838 the reactive diluents consist of esters containing reactive epoxide groups either in the fatty acid or the alcohol moiety. In these cases the reaction of the diluent with the coating system is a ring-opening reaction between the diluent and reactive hydroxyl groups in the resin and the diluent.
In U.S. Pat. No. 4,477,534 the resin contain unsaturated fatty acid esters and the reactive diluent an air drying vinyl oxazoline ester. The diluent reacts in air with the unsaturated bonds in the resin.
Fatty acid derivatives have also been reported in several patents related to heat-curable coatings. In JP 09137078, esters of unsaturated conjugated fatty acids and polyhydric alcohols are used a base for paints that claims to be heat or light curable. The curing takes place by the reaction of the unsaturated fatty acid groups. Similarly, in BE 805300 unsaturated fatty acid groups are used in the binder. The curing takes place at an elevated temperature through the reaction between the unsaturated fatty acids.
Several inventors report the use of fatty acid esters in epoxy resins in heat curing systems. JP 08325509 use a thermosetting water-dispersible epoxy resin containing fatty acids and P-bonded hydroxyl groups to obtain a coating suitable for metals. JP 63248869 use an epoxy resin obtained by reacting epoxy compounds with fatty acids. The resin is suitable for printing on metals. In both these reports the fatty acid is incorporated in the resin before application on the surface. The curing takes place via radical polymerisation of unsaturated prepolymers in the resin and amine-containing harderners. U.S. Pat. No. 4,962,179 reports a different approach using epoxide modified fatty acid in the form of epoxidised vegetable oils. The coating cures upon heating through the reaction between epoxide group of the fatty acid with amines in the coating formulation.
In DD 257442 the solvent content of the coating is reduced by modifying the alkyd binder with a dicyclopentadiene fatty acid ester derivative. In U.S. Pat. No. 4,100,046 the cycloalkenyl group is added to an fatty acid to obtain a heat-curable binder. In both these report the fatty acid is incorporated in the resin prior to application and not used as diluent in the system. The curing reaction takes place by the reaction of unsaturated cycloalkenyl groups.
In JP 2000212483 a polyglycerin ester of fatty acids is added to an aqueous paint. The ester does not reduce the viscosity (i.e. act as a diluent) or take part in the curing of the coating.
Fatty acid esters have also been used in heat curable powder coatings. In NL 1009254 unsaturated fatty acid ester is used to improve adhesion and appearance after curing. In JP 06345822 a fluorine powder resin contains esters of fatty acid and unsaturated vinyl alcohol. In neither of these examples the fatty acid ester act as a diluent since the coatings are solids and not liquids.
These examples demonstrate the need to find ways of reducing the use of solvents in coatings. It also shows the great interest in fatty acid esters as an ingredient in the pain formulations. When fatty acids are used as the reactive part of the formulations it is either as unsaturated compounds capable of curing with the air, or as epoxides reacting with other components in the formulation.